Since a beautiful surface texture is required for stainless steel articles, in the production of stainless steel formation of surface defects should be controlled as far as possible. However, when stainless steel is hot rolled by means of a tandem mill, a very small surface part of the material being rolled may be picked up by rolls and transferred to the entity of the material. Repetitions of such phenomena may frequently result in roll scoring and in turn surface defects of a hot rolled product, which are brought to the subsequent cold rolling step and deteriorate a surface quality of a cold rolled product. The surface defects of the cold rolled product, even if they are of a slight extent, not only limit application of the product, for example, make the product unsuitable for use in mirror finished applications, but also require to be remedied by polishing for use in other applications. The surface defects of a great extent invite an economical loss, since the product is only useful as a scrap.
To solve the problem of surface defects of stainless steel products caused by rjoll scoring, several approaches have heretofore been made. In one approach, rolling conditions, including reduction in rolling load have been studied. However, up to now no satisfactory solution has been obtained by this approach.
In another approach, roll material has been studied and there has been proposed that use of grain rolls having a suitable surface roughness in hot rolling of stainless steel is effective to prevent roll scoring. However, if such a grain roll is used in an initial stage of the hot rolling step where a rolling load is large (that is in the first to third stands of a hot rolling mil), the roll is considerably worn, resulting in a stepped wear, whereby a freedom of inverse rolling which is indispensable in the modern rolling technology is lost. Furthermore, with mild steels grain rolls generally provide more or less worse surface texture. Accordingly, use of grain rolls does not provide a complete solution in the the modern rolling technology where schedule free rolling is intended or where rolling of various steel species with one and the same mill is intended.
In a further approach, use of lubricant oils has been studied. The presence of a lubricant oil between the rolling rolls and the material to be rolled effectively lowers the friction coefficient, and thus, use of lubricant oils is fairly prevailing in rolling of mild steels. Similar effect is expected in the case of stainless steels and use of lubricant oils has been considered to be effective in hot rolling of stainless steels to prevent roll scoring. However, with known animal and vegetable fats and oils, and known mineral and synthetic lubricants, satisfactory effects to prevent roll scoring are not achieved in hot rolling of stainless steels.
On the other hand, for the purposes of schedule free rolling or rolling of various steel species with one and the same mill, it is frequently desirable to use high chromium rolls as work rolls in initial stages of hot rolling where a rolling load is fairly large. However, high chromium rolls only pose a more serious problem of roll scoring when used in hot rolling of stainless steel, since they have a chemical composition similar to the rolled material.
In any event occurrence of roll scoring not only renders the surface quality of the products poor but also invites problems of increase of costs and time for roll exchange.